Project Summary Posttraumatic osteoarthritis (PTOA) is a common and serious complication that can occur following acute knee injury, such as anterior cruciate ligament (ACL) rupture. The inability of surgical reconstruction to prevent the development of PTOA, suggests that acute biochemical changes in the joint following injury trigger the cascade of events leading to PTOA. Perpetuation of the inflammatory response to injury is hypothesized as an important factor in the process of joint degeneration, thus representing an attractive target for therapy. In general, the inflammation observed in PTOA is chronic, low-grade, mediated primarily by the innate immune system. Although several mechanisms have been identified that can activate the inflammatory response in vitro, there is little known on how the inflammatory response occurs in vivo, since we lack the technology to track inflammation in vivo. Current methods to assess inflammation in PTOA are either destructive like histology, are not tissue-specific like the analysis of inflammatory factors, or lack the biological specificity such as imaging. Molecular imaging represents a natural approach to address the pathogenesis of PTOA, since it can image inflammation processes at the molecular level with complete joint coverage. Our research aims to develop a new contrast agent to image low-grade inflammation in PTOA in vivo by using a peptide that specifically target cell surface receptors, which play important roles in inflammatory signaling and tissue repair. We propose to use a multimodal imaging approach combining magnetic resonance imaging (MRI) and near infrared (NIR) imaging. We will conduct experiments to validate the contrast agent in vitro and to assess its pharmacokinetics at early and late stage in vivo. Lastly, we will test the contrast agent in a longitudinal study. The results of this proposal will provide a platform to elucidate the pathophysiology of PTOA, help identifying new therapeutic targets, and track therapy response. The molecular probe developed in this application has potential for human clinical translation since its targets are preserved cross-species.